Missing Attributes in Update Merge Request API

[armsnyder]

There are a number of attributes in the Update Merge Request API which cannot be set using the existing parameters of the MergeRequestApi#updateMergeRequest method.

The full list of accepted attributes can be found here:
https://docs.gitlab.com/ee/api/merge_requests.html#update-mr

[gmessner]

@armsnyder
Your PR has been released in release 4.8.27. Thanks again for your contribution.

[armsnyder]

@gmessner Thanks for the fast turnaround!